1. Field of the Invention
This invention relates to a seal assembly, and in particular to a seal assembly suitable for effecting a seal between two machine components.
2. Description of the Related Art
It is frequently necessary to effect a fluid-tight seal between two machine components which may move one with respect to the other. For example, it may be necessary to effect a seal between a housing having a bore and a shaft which extends through the bore; or to effect a seal between the two components of a ball joint. There have been innumerable proposals for seal devices adapted to effect such seals between two relatively-movable machine components, and it is necessary for a machine designer to select the most appropriate type of seal device, having regard to the operating conditions which may be expected to be encountered.
In the case of machine components which are to be sealed one to the other in the presence of pressurised hot gases, it is known to use a brush seal assembly, on account of the unsatisfactory performance of a simpler seal having for example a synthetic elastomeric annular sealing member clamped to one of the machine components and bearing on the other component. In the case of a brush seal, a plurality of fine bristles (such as of bronze, chrome-nickel alloy or other alloy wires) is held in a carrier mounted on one of the machine components such that the tips of the bristles wipe against the other machine component, so as thereby to effect a seal between the two components.
It is a characteristic of the known designs of brush seal that they must be manufactured to very close tolerances, in order to achieve an effective seal. If the bristles are too short, there will be a gas leakage path between the tips of the bristles and the other machine component against which the seal is to be effected; but on the other hand, if the bristles are too long there will be excessive friction between the bristles and the other machine component, leading to rapid wear of the other machine component. The flexing of the bristles moreover may destroy the effectiveness of the seal, giving a gas leakage path between the bristles.
In addition to the brush seal being manufactured to close tolerances, it is apparent that the machine components also must run true, within fine limits. Whilst this is normally not a problem where for example a shaft is to be sealed to a housing through which the shaft extends, it can be a problem in the case of relatively movable fabricated machine components, which may not be circular. For example, if a seal is to be effected between an adjustable outlet nozzle of a jet engine and the main housing of that jet engine, it may be impractical to manufacture the nozzle and/or the housing to tolerances closer than 1 mm, especially if the nozzle diameter is relatively large. In such a case, one function of a seal device disposed between the components is to take up the manufacturing tolerances, whilst still maintaining a seal between the two components.
It is a principal object of the present invention to provide a brush seal suitable for use between two machine components to effect a seal therebetween, even when the clearance between the two machine components cannot be guaranteed to lie within a closely-controlled tolerance range.